1. Field of the Invention
This invention relates to an improved process for removing carbon dioxide containing acidic gases from normally gaseous mixtures containing them and more particularly relates to a process of accomplishing substantial removal of carbon dioxide containing acidic gases from normally gaseous mixtures by contacting the normally gaseous mixtures with a concentrated solution containing a sterically hindered amine and a tertiary amine.
2. Description of the Prior Art
It is well known in the art to treat gases and liquids, such as mixtures containing acidic gases including CO.sub.2, H.sub.2 S, SO.sub.2, SO.sub.3, CS.sub.2, HCN, COS and oxygen and sulfur derivatives of C.sub.1 to C.sub.4 hydrocarbons with amine solutions to remove these acidic gases. The amine usually contacts the acidic gases and the liquids as an aqueous solution containing the amine in an absorber tower with the aqueous amine solution contacting the acidic fluid countercurrently.
The acid scrubbing processes known in the art can be generally broken into three (3) categories.
The first category is generally referred to as the aqueous amine process where relatively concentrated amine solutions are employed during the absorption. This type of process is often utilized in the manufacture of ammonia where nearly complete removal of the acid gas, such as CO.sub.2, is required. It is also used in those instances where an acid gas, such as CO.sub.2, occurs with other acid gases or where the partial pressures of the CO.sub.2 and other gases are low. Aqueous monoethanolamine (MEA) is widely used in this type of process since it provides a high degree of CO.sub.2 removal. One of its drawbacks is the high stability of its carbamate which causes its regeneration to be slow and incomplete.
A second category is generally referred to as the aqueous base scrubbing process or "hot pot" process. In this type of process a small level of an amine is included as an activator for the aqueous base used in the scrubbing solution. This type of process is generally used where bulk removal of an acid gas, such as CO.sub.2, is required. This process also applies to situations where the CO.sub.2 and feed gas pressures are high. In such processes, useful results are achieved using aqueous potassium carbonate solutions and an amine activator.
A third category is generally referred to as the nonaqueous solvent process. In this process, water is a minor constituent of the scrubbing solution and the amine is dissolved in the liquid phase containing the solvent. In this process, up to 50% of the amine is dissolved in the liquid phase. This type of process is utilized for specialized applications where the partial pressure of CO.sub.2 is extremely high and/or where many acid gases are present, e.g., COS, CH.sub.3 SH and CS.sub.2.
The present invention pertains to an improved process for practicing the first category of acid scrubbing processes described above, namely, the aqueous amine process where relatively concentrated amine solutions are employed during the absorption. Many industrial processes for removal of carbon dioxide containing acidic gases use regenerable aqueous solutions of amines which are continuously circulated between an absorption zone where the acidic gases including the carbon dioxide are absorbed and a regeneration zone where the aqueous amine containing absorption solution which is saturated with the acidic components is desorbed usually by steam stripping. The capital cost of these acid scrubbing processes is generally controlled by the size of the absorption and regeneration towers, the size of the reboilers for generating stripping steam, and the size of the condensers which condense spent stripping steam so that condensate may be returned to the system to maintain proper water balance.
The cost of operating such scrubbing plants is generally related to the amount of heat required for the removal of a given amount of acid gas, e.g., thermal efficiency, sometimes expressed as cubic feet of acid gas removed per pound of steam consumed. Means for reducing the costs in operating these industrial processes have focused on the use of absorbing systems or combinations of chemical absorbents which will operate more efficiently and effectively in acid gas scrubbing processes using existing equipment.
There are a number of patents which describe improvements to improve the efficiency of the above-described processes for removing acidic gases from gaseous mixtures. Some of these improvements are described below.
Canadian Patent No. 619,193 teaches the use of various aqueous solutions containing salts of aminoacids for removing carbon dioxide containing gases from gaseous mixtures. This patent is concerned with the same type of acid gas scrubbing process as provided by the instant invention. However, this Canadian patent does not disclose the use of aqueous solutions containing amines wherein the amino group is sterically hindered, and particularly the amino alcohols.
Prior art workers have taught that sterically hindered amines would have low rates of combination with CO.sub.2 and apparently concluded, although other explanations are possible, that such sterically hindered amines would be inefficient in CO.sub.2 scrubbing processes. For example, Sharma, M.M., Trans. Faraday Soc., 61, 681-8 (1965) described the kinetics of reaction between CO.sub.2 and COS with 38 amines, some of which are sterically hindered amines. Other researchers have attributed relatively poor absorption rates of CO.sub.2 by amines to steric hindrance. See, for example, J. L. Frahn and J. A. Mills, Aust. J. Chem., 17, 256-73, 263 (1964) and M. B. Jensen, Acta Chemica Scandinavica, 11, 499-505 (1957).
Shrier and Danckwerts, Ind. Eng. Chem. Fundamentals, 8, 415 (1969) discussed the use of amines as promoters for aqueous carbon dioxide absorption solutions. However, these researchers only ran initial absorption rate experiments and did not recognize the unique capacity advantages obtained by using sterically hindered amines in an acid gas scrubbing process. Also of interest is Danckwerts and Sharma, The Chemical Engineer, Oct. 1966, pp 244-280.
U.S. Pat. No. 2,176,441 to Ulrich et. al. teaches the use of aminoacids having a primary, secondary or tertiary amino group and at least two nitrogen atoms to remove acidic gases. The patentees provide various general formulae for the aminoacids taught to be useful in the acid gas scrubbing process. While certain "sterically hindered amines" can be derived by proper choice of substituent groups in the general formulae there is no teaching that these amines will achieve any unexpected results, such as improved regeneration rates coupled with high rates of absorption.
There are a number of patents which disclose the use of various amines as "activators" in an alkaline scrubbing solution wherein the primary absorbent is an alkaline salt such as potassium carbonate. Some of these processes are described in U.S. Pat. Nos. 2,718,454, 3,144,301, 3,637,345, 3,793,434, 3,848,057, 3,856,921, 3,563,695, 3,563,696 and 3,642,430, as well as some other patents such as Belgian Pat. No. 767,105; British Pat. Nos. 1,063,517, 1,218,083, and 1,305,718. British Patent No. 1,238,696 describes a process whereby acid gases are removed from gaseous streams by use of alkanol amines and an organic solvent.
U.S. Pat. No. 3,856,921 is of interest since it discloses and claims the use of 2-methyl-aminoethanol, 2-ethylaminoethanol, morpholine, pyrrolidine and derivatives thereof as activators for the basic salt of an alkali or alkaline earth metal. However, this patent does not teach the use of sterically hindered amines and/or their use in an aqueous amine acid gas scrubbing process as instantly claimed.
In the prior art processes discussed above, it is apparent that the efficiency of processes employing absorbing solutions is generally limited by the relatively slow rate of transfer of molecules of the acid gas from the gas phase to the liquid phase as well as in the regeneration of the absorbing solution. Many of the above-described prior art processes deal with means to render the acid gas scrubbing process more efficient.
In copending U.S. application Ser. No. 590,427, filed June 26, 1975, the disclosure of which is incorporated herein by reference, there is disclosed and claimed sterically hindered amine compositions useful for scrubbing acid gases. These sterically hindered amines, unexpectedly improve the efficiency, effectiveness and working capacity of the acid gas scrubbing processes in all three of the above-mentioned process categories. It was postulated in U.S. Ser. No. 590,427 that the increase in cyclic capacity observed with the sterically hindered amines is due to the instability of their carbamates. In that respect, sterically hindered amines are similar to tertiary amines. Tertiary amines are not used on a commercial scale for carbon dioxide containing acid gas scrubbing due to their low rates of absorption and desorption.
It has been observed that some of the preferred sterically hindered amines for the aqueous amine process, i.e., the sterically hindered amino alcohols cannot be used in high concentrations without running the risk of precipitation when the aqueous solution is saturated with carbon dioxide. This is an economical disadvantage because conventional amines used in the aqueous amine scrubbing process, i.e., monoethanolamine (MEA) and diethanolamine (DEA) can be used in concentrations up to 5 M.
It has now been unexpectedly discovered that aqueous solutions containing the sterically-hindered amino alcohols in combination with tertiary amino alcohols are capable of achieving higher total amine concentrations than the sterically hindered amino alcohols alone. As a consequence of this unexpected discovery, more carbon dioxide containing acid gas can be treated per unit of volume of solution than with the solutions containing the sterically hindered amino alcohols alone, yet the advantages of improved cyclic capacity are still enjoyed.
It has also been discovered that the combination of tertiary amino alcohol and the sterically hindered amino alcohol provides a lower heat of reaction than the aqueous solutions containing the sterically hindered amino alcohols alone, and in some instances a lower heat of reaction than with diethanolamine can be accomplished.